In an automatic transmission for a vehicle, a shift valve is normally a solenoid valve. The shift valve is switched by operating the solenoid so as to engage or disengage frictional engagement elements. With this, a shift stage is switched. In this automatic transmission, when a failure is generated in the shift valve, the selected shift stage is not attained. Accordingly, when the generation of the failure is sensed, a control (failsafe control) to suppress the failure influence is performed. Besides, it is necessary to identify causes of the failure for performing appropriate failsafe control.
In a patent document 1, the gear ratio deviation is sensed from the selected shift stage, and the sensed actual gear ratio. Moreover, when the shift valve which is failed, and which is presumed to be the cause of the gear deviation can be presumed, the solenoid configured to control the actuation of the presumed shift valve is forcibly switched to the OFF state during the predetermined time period. It is checked whether or not that cause is the failure of the solenoid. When the failure of the solenoid is confirmed, it is possible to perform the failsafe control according to that.